CAPP

CNC Technology and Programming

MEC326T
Lecture 33

• Dr. Senthilkumaran
• Manufacturingportal.in
 Process Planning
 
Process planning is also called: manufacturing planning, process planning,
material processing, process engineering, and machine routing.

Process Planning: is the function within manufacturing facility that establishes

which processes and parameters are to be used to convert a part from its initial
form to a final form predetermined in an engineering drawing.
 
Process planning can be defined as: the act of preparing detailed work
instructions to produce a part.
Design Machine
Tool

Process
Planning

Scheduling and Production Control

The person who develops the process plan for producing a workpiece is often called
the process planner.

 The functions included in process planning are:

  - Raw material preparation
- Processes selection
- Process sequencing
- Machining parameter selection
- Tool path planning
- Machine selection
- Fixture selection
Factors affecting process Plan selection
• Shape
• Tolerance
• Surface finish
• Size
• Material type
• Quantity
• Value of the product
• Urgency
• Manufacturing system itself
• etc.
There are two approaches to carrying out the task of process planning:
  1- Manual Process Planning
2- Computer Aided Process Planning (CAPP).:
VARIANT
GT based
Computer aids for editing
Parameters selection
GENERATIVE
Some kind of decision logic
Decision tree/table
Artificial Intelligence
Objective-Oriented
Still experience based
AUTOMATIC
Design understanding
Geometric reasoning capability
 Manual Process Planning
 
In order to prepare a process plan, a process planner has to have the following
knowledge:
1. Ability to interpret an engineering drawing.
2. Familiarity with manufacturing processes and practice.
3. Familiarity with tooling and fixtures.
4. Know what resources are available in the shop.
5. Know how to use reference books, such as machinability data
handbooks.
6. Ability to do computations on machining time and cost.
7. Familiarity with the raw materials.
To prepare a process plan, the following are some steps that have to be taken:
•Study the overall shape of the part, to identify features and all critical dimensions.
•Thoroughly study the drawing. Try to identify all manufacturing features and notes.
•Determine the best raw material shape to use if raw stock is not given.
•Identify datum surfaces. Use information on datum surfaces to determine the setups.
•Select machines for each setup.
•Determine the rough sequence of operations necessary to create all the features for each
setup.
•Sequence the operations determined in the previous step. Check whether there is any
interference or dependency between operations. Use this information to modify the
sequence of operations.
•Select tools for each operation. Try to use the same tool several operations if possible.
Keep in mind the trade-off on tool-change time and estimated machining time.
•Select or design fixtures for each setup.
•Evaluate the plan generated thus for and make necessary modifications.
•Select cutting parameters for each operation.
 PROCESS PLAN
• Also called : operation sheet, route sheet, operation planning summary, or
another similar name.
• The detailed plan contains:
route
processes
process parameters
machine and tool selections
fixtures
• How detail the plan is depends on the application.
• Operation: a process
• Operation Plan (Op-plan): contains the description of an operation, includes
tools, machines to be used, process parameters, machining time, etc.
• Op-plan sequence: Summary of a process plan.
 Example process plans
Route Sheet by: T.C. Chang

Part No. S1243

Part Name: Mounting Bracket
workstation Time(min)
1. Mtl Rm
2. Mill02 5 Detailed plan
3. Drl01 4
4. Insp 1
PROCESS PLAN ACE Inc.

Part No. S0125-F Material: steel 4340Si

Part Name: Housing
Original: S.D. Smart Date: 1/1/89 Changes: Date:
Rough plan Checked: C.S. Good Date: 2/1/89 Approved: T.C. Chang Date: 2/14/89

No. Operation Workstation Setup Tool Time

Description (Min)

10 Mill bottom surface1 MILL01 see attach#1 Face mill 3 setup

for illustration 6 teeth/4" dia 5 machining
20 Mill top surface MILL01 see attach#1 Face mill 2 setup
6 teeth/4" dia 6 machining
30 Drill 4 holes DRL02 set on surface1 twist drill 2 setup
1/2" dia 3 machining
2" long
 Study of the Part Drawing
 
The part drawing provides the following features:
•Basic form and size of the workpiece.
•Outer envelop of the workpiece.
•Part features (shapes) to be produced.
•Dimensional and geometric tolerances.
•Required surface finish (roughness).
•Datum surfaces for setup and measurement.
•Material of the workpiece.

The information needed to prepare part drawing for NC include:

•The type of coordinate systems offered by the machine tool.
•Machine datum systems.
•Selection of the program datum (zero).
•Machine envelope.
•Machine operation capacity (spindle speed, feed, tolerance, and
accuracy).
  
  Datum Selection
A datum is a specific surface, line, plane, or other feature that is assumed to be
perfect and is used as a reference point for dimensions or features.
A workpiece datum: can be defined as point, line , surface, or cylinder from
  which dimensions are referenced.
The following examples may be used as guidelines for selecting datum
surfaces:
1.      Important surfaces to the function of the workpiece or he assembly.
2.      Reference planes for mating parts of an assembly.
3.      Previously machined surfaces.
4.      Surfaces that are easy to establish at the machine tool.
5.      Surfaces that are parallel to machine movements.
 
A datum can be explicitly or implicitly indicated in the part drawing.
A datum is normally called out by an identification symbol such as:
-A-
 Considerations for Raw Material
The features of raw material have a significant effect on:
       1.         The amount of material to be removed
       2.         The ease of workholding
       3.         Machine efficiency
 The raw material of a part can be prepared from the standard commercially
available stock, or it may be a casting or forging in which the rough shape and size
have been formed. Commercially available stock appears in the following forms:
•Bars (round, square, hexagonal, octagonal, flat, triangular, and half-round)
•Plates
•Sheets and coils
•Pipes and tubes
•Structural shapes (beams, angles, tees, zees, and channels)
In preparing raw material from standard stock, the following rules should be followed:
1. Make optimal use of the stock to minimize the amount of removed or wasted material.
2. Premachine datum surfaces with a manually operated machine, if possible, to facilitate the
workholding setup.
3. Prepare the raw material drawing.
 Part Features Identification and Processes Selection
 A wide variety of manufacturing processes are used to produce a workpiece.
These processes can be classified as:
·        Casting processes
·        Forming and shaping processes
·        Machining processes
·        Joining processes
·        Finishing processes
 The machining processes include:
·        Drilling (drilling, countering, countersinking, deep-hole drilling, etc.)
·        Boring
·        Tapping
·        Milling (face milling, end milling)
·        Turning (facing, straight turning, taper turning, parting, etc.)
·        Threading.
Many features must be considered in selecting machining processes. They
include:
       1.         Part features
       2.         Required dimensional and geometric accuracy and tolerance
       3.         Required surface finish
       4.         Available resources, including NC machines and cutting tools
       5.         Cost
Part features
A part feature is the distinctive geometric form or shape to be produced from raw
material; thus it determines process type, tool types (shapes and size), machine
requirements (3-, 4-, or 5-axis), and tool path.
  
There are two types of part features:
1. Basic features: are those simple forms or shapes that may require only one
machining operation. They include holes, slots, pockets, shoulders, profiles, and
angles.
2. Compound features: are those that consist of two or more basic part features. For
example, the combined result of two holes with different diameters.
 EXAMPLE: MACHINING PROCESSES SELECTION

Select the machining processes for the part shown in the Figure. Assume that the
required dimensional accuracy and surface roughness are within the process
capability of drilling and milling operations. The four sides of the raw material
have been premachined to the required dimensions.

 
Solution
Three part features can be identified from the part drawing:
•Top flat surface
•Outer profile
•Three holes
The recommended machining processes for these features are
•Face-milling the top surface
•Rough-milling the outer profile
•Finish-milling the outer profile
•Center-drilling the three holes
•Drilling the three holes
 PROCESSES SEQUENCING
 The sequence of operations is determined by the following three
considerations:
1. Datum surfaces should be machined first if multiple workholding
setups are required. If possible, datum surfaces should be premachined
in a manually operated machine to facilitate work-piece locating and
clamping. In those cases where two or more holding setups are requi red,
rough datum surfaces are preprocessed in a manually operated machine
and then used as setup references to produce finished datum surfaces for
the final workholding. This ensures the accuracy of the finished part.
2. Surfaces with larger area have precedence. Larger surfaces tend to be
more adaptable to disturbances resulting from machining operations.
3. Feature interference should be avoided. Feature interference occurs
when the machining of one feature destroys a requirement for the
production of other features. This happens when there is interaction or
dependency between machining operations.
EXAMPLE:
PROCESSES SEQUENCING
Figure below shows a workpiece in which some features are
interrelated. The workpiece has five basic features: a through slot in
side C, two angle strips, and two through holes on strip 1 that are
perpendicular to side A. The compound features are two tapped
holes perpendicular to strip 1. Develop the process sequence for
producing the part.
Solution
The raw material is cut from a block stock with dimensions 6.25 x 4.25 x 2.25
in. Studying the part features reveals that the two through holes on strip 1
interact with the formation of angle  and the slot in side C interacts with
the cutting of angle  . Machining angle strip 1 first will cause difficulty in
drilling the two holes, so the two holes must be produced before angle strip
1. Likewise, making angle strip 2 first will cause difficulty in setting up the
workpiece to produce the through slot, so the slot has to be machined before
angle strip 2 is made.
 
The recommended processes sequence is described below:
1. Setup A for machining side B
2. Setup B for machining sides A and E as well as drilling two holes on Side A
3. Setup C for machining sides C and F as well as cutting the slot in side C
4. Setup D for cutting, angle strip 1, drilling two tap holes, and tapping the two
holes
5. Setup E for cutting angle strip 2
Two approaches for computer-aided process planning
have been pursued:
1. Variant CAPP method
2. Generative CAPP method
 
The variant approach uses library retrieval procedures to
find standard plans for similar components. The standard
plans are created manually by process planners.
 
The generative approach is considered more advanced as
well as more difficult to develop. In a generative process-
planning system, process plans are generated
automatically for new components without referring to
existing plans.
A typical process planning system
Process planning modules and
databases
Process planning is the critical bridge between design and
manufacturing (Figure 3). Design information can be translated
into manufacturing language only through process planning.
Today, both computer-aided design (CAD) and manufacturing
(CAM) have been implemented. Integrating, or bridging, these
functions requires automated process planning.

Figure 3. Process planning bridges design and manufacturing.

Figure 4. The preparatory stage.
Figure 5. The production stage
 Variant CAPP
Problems associated with the variant approach
1. The components to be planned are limited to similar components
previously planned.
2. Experienced process planners are still required to modify the standard
plan for the specific component.
3. Details of the plan cannot be generated.
4. Variant planning cannot be used in an entirely automated manufacturing
system, without additional process planning.

Advantages of the variant approach

1. Once a standard plan has been written, a variety of components can be
planned.
2. Comparatively simple programming and installation (compared with
generative systems) is required to implement a planning system.
3. The system is understandable, and the planner has control of the final
plan.
4. It is easy to learn, and easy to use.
 AN EXAMPLE OF A VARIANT PLANNING SYSTEM
 
An example illustrates the step-by-step construction of a variant process-planning
system. A simplified coding system is used.
 
The code table is shown in Table 1. Because it is overly simplified for illustration
purposes, this system lacks detail and is not appropriate for actual application.
However, it is sufficient to represent the principles of coding for process planning.
We call our coding system S-CODE (Simple CODE) and the process-planning
system VP (Variant Planning).
 
VP is used in a machine shop that produces a variety of small components. These
components range from simple shafts to delicate hydraulic-pump parts. We discuss
the construction of VP in the following sequence:
 
1. Family formation
2. Database structure
3. Search algorithm
4. Plan editing
5. Process-parameter selection
1. Family Formation
In a process-planning system, family formation is based on production parts
or, more specifically, their manufacturing features. Components requiring
similar processes are grouped into the same family. In GT, different ways to
group parts into families are presented. Similar methods can be used for a
variant process-planning system.
 
The family matrix must then be represented in a manner that is consistent
with the S-CODE. A part-family matrix is a binary matrix similar to a PFA
matrix.
 
We can use Plij to represent a part-family matrix for family l, i= 1, . . . , I,
where I is the number of attributes in each code position, and j = 1, J,
where J is the number of digits (code length).
 In the S-CODE, I is equal to 8 and J is equal to 4. Plij implies that,
for part family l, code position j is allowed to have a value i.
 
A part-family matrix can be constructed in the following manner. Let
Cklj be the value of code position j for component k in family l, k = 1, . . .
, K (K is the number of components).
For k : = 1 to K
For j: = 1 to J
i=Cklj
P1ij=1
enddo
Enddo

Using this procedure, we can obtain a part-family matrix for family

1 (Figure 6) Thus far, we have a complete set of OP code sequences, OP
plans, and a family matrix. The next step is to store them in a computer-
interpretable format so that the information can be used later for new
components.
Table 1. S-CODE
Figure 6. A part-family matrix
Figure 8. Data record content
Figure 9. A database structure.
Figure 10. VP system
data
 The Generative CAPP Method

Generative process planning can be defined as a system

that synthesizes process information in order to create
a process plan for a new component automatically.
 Or regular
Either modeller with
feature feature Raw
based extractor material
design selection
data Feature with Raw material
Process
location selection
knowledge
(position and
base
orientation)

The Process Fixturing

generative selection knowledge
base

process Fixturing Tooling

method knowledge
planning base
Test
system selection Merchantability
database

Cutting
parameters Route
planner
Cutter
path

NC
machine
 ADVANTAGES OF THE GENERATIVE APPROACH

1. Generate consistent process plans rapidly;

2. New components can be planned as easily as existing
components;
3. It has potential for integrating with an automated manufacturing
facility to provide detailed control information.

Successful implementation of this approach requires the

following key developments:
• Process planning knowledge must be identified and captured.
• The part to be produced must be clearly and precisely
defined in a computer-readable format.
• The captured process planning knowledge and the part
description data must be incorporated into a unified
manufacturing database.
Major components of Generative CAPP

(i) part description

(ii) manufacturing databases
(iii) decision making logic and algorithms
PRODUCT REPRESENTATION

Geometrical information
Part shape
Design features
Technological information
Tolerances
Surface quality (surface finish, surface integrity)
Special manufacturing notes
Etc.
"Feature information"
Manufacturing features
e.g. slots, holes, pockets, etc.
INPUT REPRESENTATION SELECTION

• How much information is needed?

• Data format required.
• Ease of use for the planning.
• Interface with other functions, such as, part programming, design,
etc.
• Easy recognition of manufacturing features.
• Easy extraction of planning information from the representation.
WHAT INPUT REPRESENTATIONS

• GT CODE
• Line drawing
• Special language
• Symbolic representation
• Solid model
• CSG
• B-Rep
• others?
• Feature based model
 SPECIAL LANGUAGE CIMS/PRO REPRESENTATION
AUTAP

1.2
1 +.001
-.001
X

K5 10 CYLINDER/3,1/
11 DFIT/K,5/ a2 a3
3 2.5 12 CHAMFER/.2,2.6/ a4
a1
20 CYLINDER/2.5,1.2/ a5
t
21 LTOL/+0.001,-0.001/ sweep
direction
Y a6 Z

.2x2.6
 GARI REPRESENTATION

0 1.
0 3.0
F2
.5
2
F1
F3

3.0 Y
X

(F1 (type face) (direction xp) (quality 120))

(F2 (type face) (direction yp) (quality 64))
(F3 (type face) (direction ym) (quality rough))
(H1 (type countersunk-hole) (diameter 1.0)
(countersink-diameter 3.0)
(starting-from F2) (opening-into F3))
(distance H1 F1 3.0)
(countersink-depth F2 H1 0.5)
49